Surface cleaner

ABSTRACT

The present invention relates to methods of use for photocatalytic cleaning compositions and photocatalytic cleaning compositions effective to degrade soils deposited on a surface, to reduce the accumulation of soils on a surface, and to act as an antimicrobial agent.

This application is a divisional application of U.S. patent applicationSer. No. 10/168,933, filed Nov. 5, 2002 now abandoned, which is the U.S.National Phase application of International Application No.PCT/IB00/001950, filed Dec. 22, 2000, and claims priority of BritishPatent Application No. 9930248.1, filed Dec. 22, 1999. These threeapplications are incorporated herein by reference.

The present invention relates to sensitising agents and compositionseffective to degrade soils deposited on a surface, methods employingsaid agents and compositions, and uses thereof.

Cleaning compositions intended for general and specific uses are wellknown in the art. Such compositions will normally comprise one or moresurfactants, solvents, thickening agents, abrasive particles andcolouring agents. Although these compositions are effective at removingsoils, inevitably resoiling occurs after cleaning and thus recleaning isrequired.

A means to reduce the frequency of cleaning and recleaning would thus beadvantageous. In addition it would be beneficial if one could reduce therate of accumulation of surface soils in the first instance. The presentinvention seeks to address these problems.

The photocatalysed degradation of organic environmental pollutants inthe presence of a semiconductor such as titanium dioxide or zinc dioxideis well known (Ollis et al., Environ. Sci. and Technol., 12 (1991) 1522;Heller Am. Chem. Res. 28 (1995), 503). However, the chemicalcharacteristics of these semiconductors necessitate excitation of thesemetals in the ultraviolet region of the spectrum in order for thedegradation of the pollutants to occur. This requirement therefore makesthe use of photocatalysed degradation of soils on surfaces within aresidential environment both potentially hazardous and impractical.

The present inventors have found, however, that the use of a sensitisingagent in addition to the light absorbing material reduces the amount ofenergy required to be absorbed by said light absorber in order forcharge separation to take place, and subsequently for the photocatalyseddegradation of surface soils to occur. The present inventors have foundthat ambient light, for example sunlight or artificial light issufficient in the presence of a sensitising agent and a light absorbingmaterial to induce such a degradation.

The present inventors have found, in addition, that the use of highlyconjugated heterocyclic complexes such as polypyridine, macrocycle orphthalocyanines with various centrally coordinated atoms such as Ru, Feand Si can be used to sensitise a light absorbing agent (such astitanium dioxide or zinc oxide) not only when the light absorbing agentis coated onto a surface, but also when the agent is in solution. Thismakes the use of a light absorbing agent in conjunction with certainmetal complexes in solution ideally suited for applying to a surface toprovide a residue which will photocatalyse the decomposition of surfacesoils, and will also reduce the rate of accumulation of soils.

Previously, the photosensitisation of nanocrystalline titanium dioxide(TiO2) films by polyimide bearing pendant substituted-Ru (bpy)3+2 groupshas been reported (Osora et al, J. Photochem. Photobiol. B: Biol. 43(1998) 232). In this study it was found that these photosensitisedcomplexes could degrade methylene blue.

Additionally, the use of metal complexes as photosensitisers inelectrochemical cells is well established (Kalyanasundaram et al. Photo.Sens. And Photocat Using Inorg. And Organomet. Comps, 247-271). Now theuse of particulate semiconductors such as TiO2 with sensitisers isresulting in the development of a new class of solar cell (Graetzel etal, Nature, 1991, v353, 737).

Graetzel et al (JACS V107, (1985), 2988 showed thattris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium (II) dichloride is asuperior sensitising agent for charge injection into titania at acid pHcompared to tris(2,2′-bipridyl)ruthenium (II) dichloride due to theformer having carboxylate anions capable of binding with titania underacid conditions. In this same work it was also shown that thesensitising properties of tris(2,2′-bipridyl)ruthenium (II) dichlorideimprove at pH 7 compared with lower pH.

Bendig et al (J Photochem Photobiology A: Chemistry 108 (1997) 89),describe the sensitised photocatalytic oxidation of herbicides usingtris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium (II) dichloride,tris(2,2′-bipridyl)ruthenium (II) dichloride and a methylated form ofthe latter. Bedig et al. showed that onlytris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium (II) dichloride isactive under acid conditions (pH 3) under their experimental conditions,and the authors presented data showing thattris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium (II) dichloride desorbsprogressively as the pH is raised whereas tris(2,2′-bipridyl)ruthenium(II) dichloride is more strongly absorbed with increasing pH. Theauthors explain the results on the basis that titania has a point ofzero charge (PZC) such that the surface is positively charged belowapproximately pH 6 and negatively charged at a higher pH. Accordingly,under acid conditions sensitising agents carry a negatively chargedgroup can bind via electrostatic interaction, whereas positively chargedgroups will tend to be repelled. Conversely, at pH greater than the PZCvalue for titania, molecular moieties with positively charged groupswill tend to bind more strongly with the TiO₂ surface.

It follows therefore, that for sensitisation to be most effective at aparticular working pH, on semiconductors such as titania, zinc oxide,tin oxide etc., charged groups of the appropriate sign should be presenton the absorbing sensitiser-molecule to promote binding. Thus forapplication at pH conditions where the semiconductor material has aexcess negative charge, a sensitising molecule should preferably have apositively charged group or groups in its structure.

In the first aspect, the present invention provides a method for using acomposition comprising a photocatalyst and a metal complex sensitisercomprising a ligand with a conjugated π system which absorbs lightsubstantially in the visible and/or the infrared region of the spectrum,effective to deposit a functional residue of said composition on asurface.

The term ‘functional residue,’ in the context of the present inventionmeans a residue or layer of photocatalytic composition provided on asurface whereby soils deposited on the residue or layer or soils whichare present on the surface prior to the deposition of the residue orlayer are subject to a photocatalytic or other photochemical oxidation,reduction, free radical or other photochemical reaction effective tosubstantially break down, or otherwise decompose the soil. In effect,the cleaning process continues after the conventional act of soilremoval is completed. In addition, these reactions may also provide anongoing antibacterial effect that continues after the physical cleaningprocess has been completed. Finally, if a functional residue ofphotocatalytic material is applied to a substantially clean or sterilesurface then the rate of accumulation of soils on the surface will bereduced.

The term photocatalyic agent in the context of the present inventionrefers to an agent that has a favourable combination of electronicstructure, light absorption properties, charge transport characteristicsand excited-state lifetimes. Primary light absorbers for photocatalysisinclude but are not limited to semiconductor materials.

One model of dye sensitisation of the semi-conductor titanium dioxide,suggests that surface adsorbed dye molecules (sensitising agents) absorbvisible light and inject electrons into the conduction band thus:D+hv->D*D*+TiO2->D.++(TiO2+e-CB)The conduction band electrons may then reduce oxygen to reactive speciessuch as OH radicals, which can rapidly attack organic molecules, i.e.3D30 3hv+O2+3H+->3D.+.OH+H2O on TiO2Alternatively or simultaneously D.+ may oxidise organic molecules. Inthis invention it will be understood by those skilled in the art thatthe sensitising agent is working in a catalytic manner i.e it is notsignificantly altered itself during the photocatalytic cleaning process,and is therefore active over a long period of time.

Suitable photocatalytic agents include but are not limited to titaniumdioxide (in the form of anatase and/or rutile and/or brookite), zincoxide, tin oxide, cadmium sulphide, tungsten trioxide and molybdenumtrioxide. Alternatively, combinations of two or more of these agents maybe used. In a preferred embodiment the agent is titanium dioxide.

In the present invention, the photocatalytic composition furthercomprises a metal complex sensitiser. The central atom of suchsensitisers can be but is not limited to ruthenium, platinum, palladium,iridium, rhodium, osmium, rhenium, iron or copper, titanium or zinc. Inone embodiment suitable sensitising agents include but are not limitedto heterocyclic complexes which contain polypyridine, macrocyclic orphtalocyanine ligands and optionally other ligand types wherein at leastone of the nitrogen groups is displaced by other donor groups. In apreferred embodiment of the invention the complex is any one or more ofruthenium II, III or IV or mixed oxidation state chelating complexescontaining nitrogen donor atoms or a ruthenium(II), (III), (IV) or amixed oxidation state polypyridine complex.

In a further embodiment the sensitising agent includes any one or moreof the following groups: terpyridyls, bipyridyls, phthalocyanines,porphyrins, tetra-aza-annulenes, pyrazines, phenanthrolines andderivatives thereof and compounds with substantially similar nitrogenbased ring systems.

These groups may be derivatised to produce compounds containingpositively-charged binding sites suitable for attachment tosemiconductors. Thus the sensitising agent may further include any oneor more of R₄N+ or R₄P+ groups wherein each R group may be the same ordifferent and is any one or more of the following groups: hydrogen,halogen, amine, alkyl, aryl, arylalkyl, alkoxy, heterocyclic groups, orderivatives thereof, including acid and ester derivatives, any of whichmay be branched or unbranched, substituted or unsubstituted.

In this way, sensitising agents are specifically designed wherein themolecular structure functions in combination with semiconductors wherethe desired operating condition is such that the un-coated semiconductorsurface presents adsorption sites with a negative charge. This willoccur for instance where the composition containing said agent is ofalkaline pH.

On one embodiment the sensitising agent may include a terpyridal groupof general formula I shown below:

Where at least one of R1, R2 and R3 are positively charged groups whichhas the general formula II shown below:

Where R5-R7 are any one or more of the following groups: hydrogen,halogen, amine, alkyl, aryl, arylalkyl, alkoxy, heterocyclic groups, orderivatives thereof, including acid and ester derivatives, any of whichmay be branched or unbranched, substituted or unsubstituted,

Alternatively, or in addition the sensitising agent may include abipyridyl group having the general formula III shown below:

Where R8 and R9 can be the same or different and is any one or more ofthe following groups: hydrogen, halogen, amine, alkyl, aryl, arylalkyl,alkoxy, heterocyclic groups, or derivatives thereof, including acid andester derivatives, any of which may be branched or unbranched,substituted or unsubstituted,

R2 may be the same or different from R3 and is any one or more of thefollowing groups: hydrogen, halogen, amine, alkyl, aryl, arylalkyl,alkoxy, heterocyclic groups, or derivatives thereof, including acid andester derivatives, any of which may be branched or unbranched,substituted or unsubstituted,

Alternatively, or in addition sensitising agents of the presentinvention may include phtalocyanines of general formula IV below:

Where each R group may be the same or different and is any one or moreof the following groups: hydrogen, halogen, amine, alkyl, aryl,arylalkyl, alkoxy, heterocyclic groups, or derivatives thereof,including acid and ester derivatives, any of which may be branched orunbranched, substituted or unsubstituted.

Alternatively, or in addition sensitising agents may includetetra-aza-annulenes (TADAs) of general formula V shown below.

R1-R4 may be the same or different and is any one or more of thefollowing groups: hydrogen, halogen, amine, alkyl, aryl, arylalkyl,alkoxy, heterocyclic groups, or derivatives thereof, including acid andester derivatives, any of which may be branched or unbranched,substituted or unsubstituted.

Alternatively, or in addition the bipyridyl compoundstris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium (II) dichloride andtris(2,2′-bipridyl)ruthenium (II) dichloride can be dimerised usingpyrazine derivitives such as pyrazine, pyrimidine and 4,4′-bipyridyllinking ligands using procedures well known in the art. Again aspreviously discussed these will be most suitable for use in operatingconditions such that the un-coated semiconductor presents absorptionsites with a negative charge

Compositions of the present invention will most preferably be in theform of a liquid. They may also be in the form of an emulsion,suspension, or in particulate form. Preferably, the light absorbingagent will comprise no more than 50% w/v of the photocatalyticcomposition, more preferably the light absorbing agent will comprise nomore than 10% w/v of the photocatalytic composition. More preferablystill the light absorbing agent will comprise no more than 1% w/v of thephotocatalytic composition. Yet more preferably the light absorbingagent will comprise no more than 0.1% w/v of the photocatalyticcomposition. Preferably the sensitising agent will comprise no more than1% w/v of the photocatalytic composition. More preferably thesensitising agent will comprise no more than 0.1% w/v of thephotocatalytic composition.

The compositions of the present invention are effective at a whole rangeof pH values from 1 to 14. For compositions comprising sensitisingagents of the present invention which contain polypyridine, macrocyclicor phthalocyanine ligands and optionally other ligand types wherein atleast one of the nitrogen groups is displaced by other donor groups, inparticular any one or more of: sensitising agent is ruthenium II, III orIV or mixed oxidation state chelating complexes containing nitrogendonor atoms, or a ruthenium(II), (III), (IV) or a mixed oxidation statepolypyridine complex, then these compounds perform most effectively atpHs corresponding to a positive charged surface-state of thesemiconductor component e.g for titania this corresponds to a pH of lessthan 7. Thus in a preferred embodiment of this aspect of the invention acomposition comprising sensitising agents described above and alsotitania preferably has a pH of less than 7, even more preferably of lessthan 6, more preferably still of less than 5.

For compositions comprising a sensitising agent according to the presentinvention which includes any one or more of the following groups:terpyridyl, bipyridyls, phthalocyanines, porphyrins,tetra-aza-annulenes, pyrazines, phenanthralines and derivatives thereofand compounds with substantially similar nitrogen based ring systems,and may further include any one or more of R₄N+ or R₄P+ groups whereineach R group is as hereinbefore described, the preferred pH of thecomposition corresponds to the value where the semi-conductor componenthas a negatively charged surface. For titania this is pH 7 or greater.Even more preferred is a pH of greater than 8, more preferred still a pHof greater than 9.

It is also the case that even where the semiconductor component has asurface-excess of positive charge at a particular pH, negatively chargedsites for binding positively charged sensitising agents may well bepresent so that both charge types of sensitiser may effectively be used.Similarly, for systems where mixed semiconductor components e.g. titaniawith zinc oxide, are used both charge types of sensitising agents may beemployed.

In a further aspect the present invention provides a sensitising agentwhich includes any one or more of the following groups: terpyridyl,bipyridyl, phthalocyanine, porphyrins, tetra-aza-annulenes, pyrazines,phenanthrolines and derivatives thereof and compounds with substantiallysimilar nitrogen based ring systems.

The sensitising agents listed above further includes any one or more ofR₄N+ or R₄P+ where R5-R are any one or more of the following groups:hydrogen, halogen, amine, alkyl, aryl, arylalkyl, alkoxy, heterocyclicgroups, or derivates thereof, including acid and ester derivatives, anyof which may be branched or unbranched, substituted or unsubstituted.These groups may be derivatised to produce compounds containingpositively-charged binding sites suitable for attachment tosemiconductors as hereinbefore described.

In yet a further aspect, the present invention provides that use of asensitising agent according to the present invention for thesensitisation of a light absorbing agent on a surface such that soilspresent on the surface are substantially broken down and/or the rate ofaccumulation of such soils on a surface is significantly diminished.

The term ‘the rate of accumulation of soils is significantly diminished’in the context of the present invention means that the rate issignificantly diminished as compared with a similar sample in which nosensitising agent has been applied.

The photocatalytically active composition, may be doped with anadditional element which has the effect of reducing the energy requiredto promote an electron of the photochemically active material to theconductance band. Suitable doping agents may include but are not limitedto platinum, palladium, cobalt, silver, copper, nickel or iron,tungsten, chromium. These may be present as the metals themselves,and/or as complexes and/or compounds thereof.

Compositions of the present invention may further include a wettingagent which may be any one or more of the following: Igepal® CA-520[polyoxyethylene(5)isooctylphenyl ether], Igepal® CA-630[(octylphenoxy)polyethoxyethanol], Igepal® CA-730[polyoxyethylene(12)isooctylphenyl ether]. Preferably the concentrationused will be between 0.5-5.0 wt %, even more preferably between 0.5 and3 wt %, more preferably still between 0.5 and 2.0 wt %.

The photocatalytic compositions and/or sensitising agents of the presentinvention can be used in conjunction with those conventional ingredientsof cleaning materials known to those skilled in the art. These mayinclude but are not limited to water, anionic, non-ionic or amphotericsurfactants. Grease cutting, surfactant synergistic or other solventsmay also be included as may antibacterial agents, suspending agents,colourants, perfumes, thickeners, preservatives and so on. Some or allof the ingredients may be of high volatility whereby a residue ofphotochemically active material can be left behind on a surface in acontrolled manner.

The sensitising agent, or compositions according to the presentinvention may be applied to the surface in any appropriate form such as,for example, a liquid, cream, mousse, emulsion, microemulsion or gelform and may be dispensed either directly from the bottle or by means offor example an aerosol, pump action dispenser. These means will be knownto those in the art.

One skilled in the art will appreciate that generally the compositionsand/or sensitising agent according to the present invention oncedeposited on the surface should be substantially imperceptible to theuser. This may be achieved by using materials, agents and compositionswith a microscopic particle size. The microscopic particle size alsoaids in achieving a uniform dispersion throughout the materials and/orcompositions thus maximising the efficiency of the photochemicalreaction. Preferably the particle size is less than 100 nm, morepreferably the particle size is less than 50 nm and more preferablystill it is less than 20 nm

In some circumstances one skilled in the art will appreciate the needfor the photocatalytic composition and/or sensitising agent to possesslarger particle sizes.

The invention will now be described with reference to the followingexamples in which are in no way limiting of the invention, and in which:

FIG. 1 represents the UV/Visible spectra of the λ max of the target dyeGentian Violet disappearing with time as described in example 6.Horizontal axis is wavelength in nm. Vertical axis is absorbance inunits measured using UV spectrometer (UV/vis spectrometer-UV 4-UNICAM) ♦represents sensitised TiO2+0.1 ml Gentian Violet dye (GV) at T=0 mins, ▪Sensitised TiO2+0.1 ml GV T=30 mins, ▴ Sensitised TiO2+0.1 ml GV T=1hour, X Sensitised TiO2+0.1 ml GV T=2 hours, — Sensitised TiO2+0.1 ml GVT=3 hours, ● Sensitised TiO2+0.1 ml GV T=4 hours.

FIG. 2 represents the activity of TiO2 sol (sol 1) as described inexample 8. Horizontal axis represents time and the vertical axisrepresents the change in absorbance measured using a (UV/visspectrometer-UV 4-UNICAM) ♦ represents the activity of sensitised TiO2at pH 3.28, ▪ Activity of sensitised TiO2 at pH 2.08, ▴ Activity ofsensitised TiO2 at pH 2.72, X Activity of sensitised TiO2 at pH 4.02.

Feature 3 represents the activity of TiO₂ sol (sol 2) as described inexample 8. Horizontal axis represents time and the vertical axisrepresents the change in absorbance measured using a (UV/visspectrometer-UV 4-UNICAM) ♦ represents the activity of sensitised TiO₂at pH 2.00, ▪ Activity of sensitised TiO₂ at pH 2.64, ▴ Activity ofsensitised TiO₂ at pH 4.12, × Activity of sensitised TiO₂ at pH 3.39,*Activity of sensitised TiO2 at pH 5.00, ● Activity of sensitised TiO₂ atpH 5.98.

FIG. 4 represents the activity of the TiO₂ sol (sol 3) as described inexample 8. Horizontal axis represents time and the vertical axisrepresents the change in absorbance measured using a (UV/visspectrometer-UV 4-UNICAM) ♦ represents the activity of sensitised TiO2at pH 4.1, ▪ Activity of sensitised TiO₂ at pH 3.2, ▴ Activity ofsensitised TiO₂ at pH 2.7, × Activity of sensitised TiO2 at pH 2.1, *Activity of sensitised TiO2 at pH 5.2, ● Activity of sensitised TiO₂ atpH 6.0, — Activity of sensitised TiO₂ at pH 6.5-7.0.

FIG. 5 represents the activity of TiO₂ (sol 4) as described in example8. Horizontal axis represents time and the vertical axis represents thechange in absorbance measured using a (UV/vis spectrometer-UV 4-UNICAM)♦ represents the activity of sensitised TiO₂ at pH 2.74, ▪ Activity ofsensitised TiO₂ at pH 2.12, ▴ Activity of sensitised TiO₂ at pH 3.38, ×Activity of sensitised TiO₂ at pH 4.00.

FIG. 6 represents the activity of sensitised TiO₂ sol at different pH asdescribed in example 9. Horizontal axis represents time and the verticalaxis represents the change in absorbance measured using a (UV/visspectrometer-UV 4-UNICAM) ♦ represents the activity of sensitisedsolution (1) at pH 6.7, ▪ Activity of sensitised TiO₂ at pH 5.2, ▴Activity of sensitised TiO₂ at pH 8.8.

FIG. 7 represents the effect of the light source in photocatalyticactivity as described in example 11. Horizontal axis represents time andthe vertical axis represents the change in absorbance measured using a(UV/vis spectrometer-UV 4-UNICAM) ♦ represents daylight bulb 40 W, ▴daylight bulb 100 W, * fluorescent 8 W, ▪ tungsten filament 35 W, ●overhead projector, — tungsten filament 100 W.

EXAMPLES

In the following examples OHP represents overhead projector, GV standsfor Gentian Violet dye.

Example 1

A nanocrystalline titanium dioxide sol was applied to the surface of apreviously cleaned glass microscope slide by spin coating 0.5 ml of thetitanium dioxide sol at 1500 rpm for 30 seconds. The glass slide wasthen fired at 450° C. for 30 minutes. Once cool the process was repeatedtwo further times to give 3 coats of the nanocrystalline titaniumdioxide. The slide was then immersed in an aqueous 1×10⁻6M solution oftris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride) for 30 minutesto allow adsorption of the sensitising agent to the titanium dioxide.The slide was removed, rinsed with water to remove any unbound rutheniumcomplex and then stained with a 0.3% Gentian Violet solution(N-4[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadien-1-ylidenel]-N-methylmethanaminiumchloride) in 20% ethanol by immersing in the dye for 5 minutes. Onceagain the slide was washed with water to remove any unbound dye. Thewhole process was repeated once more for a second identical slide andthen twice more, but omitting immersing these two slides in thetris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride) to give twoun-sensitized control slides.

One each of the sensitised and un-sensitised slides was kept in totaldarkness. The other two slides were placed on top of an overheadprojector fitted with two 24V 250 W tungsten halogen bulbs.Decolourisation of the purple colour was monitored as the Gentian Violetwas decomposed photocatalytically. Within 50 minutes there was anoticeable difference between the colour of the sensitised andun-sensitised slides particularly when compared to the controls storedin the dark. On the slide with the sensitised titanium dioxide theGentian Violet was decomposing and hence the purple colour fading. Thiscontinued until there was no purple colour left. Decomposition of theGentian Violet did also occur on the un-sensitised slide but at asignificantly slower rate.

Example 2

A nanocrystalline titanium dioxide sol thickened with methylcellulosewas screen printed on to a series of cleaned glass microscope slides.The printed titanium dioxide films were then fired at 450° C. for 30minutes. Half of the slides were then immersed in an aqueous 1×10⁻⁶Msolution of tris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride)for 30 minutes to allow adsorption of the sensitising agent to thetitanium dioxide. The slides were then removed from the sensitisingsolution and washed with water to remove any unbound ruthenium complex.All the slides, both sensitised and unsensitised were then divided intotwo groups. One set was immersed in 0.3% Gentian Violet solution(N-4-[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadien-1-ylidene]-N-methylmethanaminiumchloride) in 20% ethanol for 5 minutes and the second set into a 0.3%aqueous solution of Acid Orange dye(4-[(2-hydroxy-1-napthalenyl)azo]-benzenesulfonic acid monosodium salt)for 5 minutes. A sensitised and unsensitised slide dyed with either theGentian Violet or Acid Orange stains was placed in total darkness andused as a control for each treatment. A second equivalent set was leftexposed to daylight next to the window of a south-facing window. A thirdand final set was also left exposed to the daylight through asouth-facing window but these slides were covered with a 6 mm thickpiece of Perspex which substantially absorbs the UV component of thelight. Decolourisation of both the purple and orange colours wasmonitored as both the Gentian Violet and Acid Orange were decomposedphotocatalytically. After 48 hours exposure to light the slides dyedwith Gentian Violet and left directly on the open bench were partiallydecolourised. The slides stored under the Perspex had begun todecolourize but at a slower rate than those not under Perspex. By day 7the dye on all the slides left just on the bench had either completelyor almost completely disappeared. The slides under Perspex reached thesame amount of decolourization on day 14. There was no change in thecolour of the slides stored in the dark. All the light exposed slideswere re-dyed with either Gentian Violet or Acid Orange and treatedexactly as before. None of the Acid Orange stained slides wouldre-stain. The sensitised titanium dioxide slides stained with GentianViolet and exposed directly to daylight decolourised completely within24 hours. The unsensitised slide had still not completely decolourised 5days after restaining. The slides under Perspex were still coloured 5days after restaining, however the sensitised slide had faded to agreater extent than the unsensitised slide.

Example 3

To 1.0 ml of a nanocrystalline titanium dioxide solution, 4.0 ml of a3.4×10⁻⁵ M aqueous solution oftris(2,2′-bipyridyl-4,4′-dicarboxylate)Ru(II)(dichloride) were added andthe resulting solution mixed well using a vortex mixer. A film of thissolution was prepared by spin coating 0.1 ml of this solution at 1500rpm on a clean glass microscope slide for 30 seconds. The film was driedusing a hand held hot air drier and the process repeated twice more togive a total of 3 coats on the microscope slide. A second slide was thenprepared in exactly the same way. Both slides were then immersed into asolution of 0.3% Gentian Violet(N-4-[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadien-1-ylidenel]-N-methylmethanaminiurmchloride) in 20% ethanol for 5 minutes. The slides were removed, rinsedwith water to remove any excess stain and allowed to air dry. One slidewas kept in total darkness and the second was placed on top of a pieceof 6 mm thick Perspex (to remove any UV light) on an overhead projectorfitted with two 24V 250 W tungsten halogen bulbs. The purple colour onthe light exposed slide steadily decomposed and after 3 hours hadcompletely faded. There was no change in the colour of the slide storedin darkness.

Example 4

Titania Sols Preparation

Kormann Method. (C. Kormann, D. W. Bahnemann, M. R. Hoffmann, J. Phys.Chem., 1988, 92, 5196)

TiCl₄ (3.5 ml) was slowly added to cold de-ionised water (900 ml) undervigorous stirring. The resulting clear solution was stirred at 0° C. for3 hours then dialysed between 2 hours and 24 hours. The clear solutionis then dried using a rotary-evaporator (Temperature of water bath=30°C.). The resulting white powder (TiO₂) is then re-suspended intode-ionised water at the desired concentration. The dialysismembrane—Visking—Size1350/1 MWCO 1350 Daltons was treated prior to use.The membrane was left for 30 minutes at 80° C. in a solution containingEDTA (1 mM) and 2% NaHCO₃. The membrane was then washed thoroughly withde-ionised water.

Method According to GB 1 412 937

An aqueous TiCl₄ solution (50 ml of TiCl₄ diluted in 500 ml de-ionisedwater) was added into a beaker containing de-ionised water (3 L) andconcentrated ammonia (40 ml) with continuous stirring. The white mixturewas stirred for about 20 minutes then allowed to settle. he supernatantwas removed using a peristaltic pump. The volume was completed again to3 L with de-ionised water, stirred then allowed to settle. Thesupernatant was removed. This process was repeated twice. The volume wascompleted with de-ionised water to 3.5 L. The mixture was stirred, thepH was checked (pH 8.8) then a nitric acid solution (1M) was addedslowly to get pH close to 3.3. The mixture was stirred for 30-45 minutesthen allowed to settle. The supernatant was removed the nitric acid (1M,23.2 ml) was added to the white mixture was stirred for about 20 minutesthen was left to age for about a week. In order to increase thepeptisation step, the mixture can be heated gently to 60-70° C. for 30minutes then allowed to settle.

Isopropoxide Route

Titanium-isopropoxide (Aldrich, 400 ml, 97%) was added rapidly to abeaker containing de-ionised water (1 L). The precipitated TiO₂ wasdecanted and washed 4 times with de-ionised water (4×500 ml) thenfiltered. The wet filtered solid was digested at 70° C. withconcentrated nitric acid (16.7 ml) and de-ionised water (volume total800 ml) for 30 min to 1 h 30 min to produce a sol.

Example 5 Synthesis ofTris(2.2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride)

RuCL₃,xH₂O (1.33 mmol Ru), 1-methyl-2-pyrrolidinone (15 ml) and2,2′-dipyridyl-4,4′-dicarboxylate (4.1 mmol) were added into a roundbottomed flask and then purged with Ar5 on N₂. The mixture was heated toreflux in the dark for 1 h 30 min. 1-methyl-2-pyrrolidinone (25 ml) wasadded to the flask and the reflux was continued for a further 2 hoursunder Ar or N₂. The mixture was allowed to cool to room temperature andkept under Ar or N₂ overnight. The dark mixture was filtered. Theresulting reddish brown solid was washed with 1-methyl-2-pyrrolidinone(2×20 ml) and diethyl ether (3×20 ml) then dried under vacuum.Yield=0.61 g. Product contained 1.2moles of 1-methyl-2-pyrrolidinone.

Example 6

Activity Test

A mixture of TiO₂ sol (1 ml, 1 g/L) andtris(2,2′-dipyridyl4,4′-dicarboxylate)ruthenium(II)(dichloride) (4 ml,c=3.4×10⁻⁵M) was stirred for about 1 minute using a rotamixer. The pHwas recorded then the target dye gentian violet (GV dissolved in 20%ethanol solution, 0.05 ml or 0.08 ml, 0.03 wt/v %) was added and themixture was stirred again. The initial colour of the sensitised TiO₂ solwas yellow. Addition of gentian violet produced a purple colour at pH 3and higher. A UV/Visible spectrum was taken at this stage. The vialcontaining the mixture was placed onto an overhead projector (2 cmheight from the glass, in order to reduce heat). A UV/Visible spectrumwas used to observe the colour change over a period of time at lamda maxof the Target dye (Gentian violet or crystal violet)=588 nm in the whitelight spectrum. (OHP used: Model Ensign. Lamp:24V-250 W-3860 lux)

In order to get quantitative data a UV/Visible spectrum was taken atdifferent times. The results are shown in FIG. 1.

Example 7

Effect of Different Steps of a Preparation, Effect of Pentisation and pHEffect.

The “activity” has been tested at different stages of the preparation ofthe TiO₂ by hydrolysis of TiCl₄ (Kormann Method-see example 4 fordetails). The effect of peptisation has also been looked at. From theresults shown in Table 1, the peptisation as well as the particle sizeseemed to have little effect on the activity.

The details of the different preparations as well as the activity testare summarised in Example 4 and 6.

TABLE 1 Effect of peptisation on activity. Time to TiO₂ sol Temp of Timefor Particle size decolourise Code Ti Source Preparation PeptisationPeptisation (nm) 0.05 ml Gv Sol A TiCl₄ Prepared as 60-65 C. for 99%after 22.5 30 min per example 1 30 mins 48 hours of GB 1412 937(Woodhead) Sol B Ti isopropoxide Isopropoxide Ambient 99% after 7 20.790 min route days Sol C Ti isopropoxide Isopropoxide 60 C. for *95%after 7 34.5 40 min route 30 mins days Sol D Ti isopropoxideIsopropoxide 70 C. for 30 min 95.4 55 min route 30 mins Sol E TiCl₄Prepared as Ambient 99% after 7 20.3 60 min per example 1 days of GB1412 937 (Woodhead) *Significant amount of non-dispersed materialpresent after 5 days so further nitric acid (1M) was added to giveNO³⁻:Ti ca 0.27 Note: the mixture was 1 ml TiO2 (1 g/L) and 3 mlsensitiser (c = 3.4 × 10⁻⁵M)

Example 7b

The pH of the sensitised sol (TiO₂ sol prepared by Kormann method) wasfound to be different at each step of the process. The results aresummarised in Table 2. The pH was measured using a pH meter (HANNAInstruments—H18424 microcomputer).

TABLE 2 Effect of different steps of the process on activity. pH of theTime to decolourise Steps of the process sensitised sol 0.08 ml GentianViolet Before dialysis 1.85 2 h 45 min After dialysis 2.95 2 h 20 minSol dried using rotary-evaporator 3.16 4 h 40 min then re-suspended Soldried using freeze-drier then 3.08 2 h 45 min mixture stil re-suspended*purple. *TiO₂ was very difficult to re-suspend. After sonication the solwas cloudy.

The results indicate that the activity may be related to pH.

Example 8

The effect of pH on activity of various sols

Four different sols have been tested at pH ranging from 2 to 7. Theyare:

-   -   (Sol 1) Hydrolysis of TiCl₄ followed by a dialysis, dried on        rotary-evaporator then re-suspended. The pH of the sol was        adjusted with HCl (1M) or NaOH (0.01M).    -   (Sol 2) Hydrolysis of TiCl₄ followed by a dialysis only. The pH        of the sol was adjusted with HCl (1M) or NaOH (0.01M).    -   (Sol 3) Precipitation of titanium-isopropoxide followed by        peptisation with nitric acid. The pH of the sol was adjusted        with HNO₃ (0.1M) or NaOH (0.01M).    -   (Sol 4) Precipitation of TiCl₄ followed by preptisation with        nitric acid. The pH of the sol was adjusted with HNO₃ (0.1M) or        NaOH (0.01M).

This experiment was carried out over a 2 hour period. All sols wereprepared at 1 g/L and the amount of Gentian Violet (0.08 ml, 0.03 wt/v%) was kept the same for each type of sol. Most of the sensitised solshad a precipitate or were precipitating. However, the systems were stillworking even in presence of a precipitate. The “activity” was reducedwith an increase of pH. The results are summarised in Table 3.

A mixture of TiO₂ sol (1 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride (4 ml,c=3.4×10⁻⁵M) was stirred for about 1 minute using a rotamixer. The pHwas adjusted then the target dye gentian violet (GV dissolved in 20%ethanol solution, 0.08 ml, 0.03 wt/v %) was added and the mixture wasstirred again. The initial colour of the sensitised TiO₂ sol was yellow.Addition of gentian violet produced a purple colour at pH 2.5 andhigher. Below pH 2.5, addition of gentian violet produced a blue-greencolour. A UV/Visible spectrum was taken at this stage. The vialcontaining the mixture was placed onto an overhead projector (2 cmheight from the glass, in order to reduce heat). A UV/Visible spectrumwas used to observe the colour change over a period of time. (OHP used:Model Ensign. Lamp:24V-250 W-3860 lux). In order to get quantitativedata a UV/Visible spectrum was taken at different times.

TABLE 3 pH effect on the activity of different type of sols. pH of Solsensitised Time to decolourise 0.08 ml Gentian Activity preparation solsViolet rank 1 2.08 The mixture was nearly yellow and High clear after 2hours. 2.72 The mixture was orange-pink with a Medium slight precipitateafter 2 hours. 3.28 The mixture was still purple with a Low precipitateafter 2 hours. 4.02 The mixture was orange-pink with a Mediumprecipitate after 2 hours. 2 2.00 The mixture was green-yellow with Highno real precipitate after 2 hours. 2.64 The mixture was orange-pink witha Medium precipitate after 2 hours. 3.39 The mixture was orange-pinkwith a Medium precipitate after 2 hours. 4.12 The mixture wasorange-pink with a medium precipitate after 2 hours. 5.00 The mixturewas purple with a Low precipitate after 2 hours. 5.98 The mixture waspurple with a Low precipitate after 2 hours. 3 2.1 The mixture wasyellow with a High precipitate after 25 minutes. 2.7 The mixture wasyellow with a High precipitate after 35 minutes. 3.2 The mixture wasyellow with a High precipitate after 1 h 30 min 4.1 The mixture wasyellow with a High precipitate after 2 hours. 5.2 The mixture was yellowwith a High precipitate after 2 hours. 6.0 The mixture was orange-pinkwith a Medium precipitate after 2 hours. 7.6 The mixture was purple witha Low precipitate after 2 hours. 4 2.12 The mixture was yellow after 30High minutes. 2.74 The mixture was yellow with a High precipitate after25 minutes. 3.38 The mixture was yellow with a High precipitate after 1h 30 min. 4.00 The mixture was yellow with a High precipitate after 2hours. The results are also shown in FIG. 2, FIG. 3, and FIG. 4, andFIG. 5.

Example 9

Addition of Stabilisers

Buffer solutions were obtained by diluting the powder buffer (BDHchemicals) into the required amount of de-ionised water.

Attempts to stabilise TiO₂ sols prepared from the hydrolysis of TiCl₄(Kormann method) were made using buffer solution pH 7 and pH 9.2.Addition of buffer solution pH 7 into a TiO₂ sol (10 ml, 1 g/L) produceda precipitate at pH 7. Addition of a buffer solution pH 7 or pH 9.2,de-ionised water and solid TiO₂ produced cloudy solutions at differentpHs (see Table 4).

TABLE 4 Buffer solutions addition effect. No Sols Particle solutionsCompositions pH size (nm) Observations 1 2 ml buffer pH 7 6.8 208 nmCloudy. No apparent 10 ml water precipitate 9-10 mg TiO₂ 2 2 ml bufferpH 7 6.9 120 nm Cloudy. No apparent 8 ml water precipitate. 9-10 mg TiO₂3 1 ml buffer pH 7 6.5 121 nm Cloudy. No apparent 10 ml waterprecipitate. 9-10 mg TiO₂ 4 1 ml buffer pH 7 5.2 189 nm Cloudy. Noapparent 9 ml water precipitate. 9-10 mg TiO₂ 5 0.5 ml buffer pH 7 5.2Cloudy. A precipitate 9.5 ml water was observed after 9-10 mg TiO₂ 1 h30 min. 6 1 ml buffer pH 9.2 8.2 Cloudy. 10 ml water 9-10 mg TiO₂ 7 2 mlbuffer pH 9.2 8.8 Cloudy. 10 ml water 9-10 mg TiO₂

Solutions (1) and (4) were found to be cloudier than solutions (2) and(3). The particle size was higher for (1) and (4) this may correspond tothe cloudiness of the solutions. After 24 hours, a slight precipitatewas observed in solutions (1) and (4).

Attempts were made to increase pH with sodium hydroxide solutionsfailed. Addition of acetylacetonate to a TiO₂ sol produced a stable solwith a pH of 2.3-2.4. Increasing the pH with a sodium hydroxide solutionproduced precipitation at pH around 7.

The activity was tested for the sensitised solution (1), sensitised TiO₂sols at pH 5.0 and 8.8 (note: the pH was increased by addition of NaOH(0.01M)). The target dye decolourised quicker at pH 5 although for allthree samples there was some target dye left not decolourised after 2hours. The results are summarised in FIG. 6.

Poly(vinyl alcohol) (PVA) was tested as a potential stabiliser for TiO₂sols. It was found that addition of a large excess or too little causedprecipitation of the sols when the pH was increased with sodiumhydroxide. PVA can be dissolved by sonication or by gentle heating inwater then can be added to a TiO₂ sol. Addition of PVA directly to aTiO₂ sol, produced a precipitate.

The activity of a sensitised sol containing PVA at pH around 3 and 7 wastested. The results show that PVA and an increase in pH slowed down theactivity.

Example 10

TiO₂:Sensitiser Ratio Effect

The ratio TiO₂:Sensitiser or TiO₂:Ru has been looked at for a particularTiO₂ sol (Kormann method, TiO₂ sol dialysed only). The sol tested wasobtained from hydrolysis of TiCl₄ followed by a dialysis. The experimentinvolved variation of ruthenium and kept the TiO₂ fixed. The target dyewas decolourised quicker in 1:6 TiO₂:Ru ratio than in 1:2 TiO₂:Ru ratio

The target dye gentian violet (0.05 ml, 0.03 wt/v %) was decolourisedwithin 3 hours in 1:6 TiO₂:Ru ratio whereas in 1:4 TiO₂:Ru ratio and 1:2TiO₂:Ru ratio the gentian violet decolourised within 4 and 5 hours,respectively.

Example 11

Light Source Effect.

A mixture of TiO₂ sol (1 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride (3 ml,c=3.4×10⁻⁵M) was stirred for about 1 minute using a rotamixer. Additionof gentian violet (0.05 ml, 0.03 wt/v %) produced a purple colour. AUV/Visible spectrum was taken at this stage. The vial containing themixture was placed onto an overhead projector (2 cm height from theglass, in order to reduce heat) or in a light box. UV/Visible spectrawere used to observe the colour change over a period of time. Normallyall the activity work was done using an overhead projector as a lightsource. Other light sources such as daylight bulbs (40 W and 100 W),tungsten filament tube (35 W), tungsten filament bulb (100 W) andfluorescent tube (8 W) have also been investigated during this study.The results showed that the process still works with all light sourcestried albeit much slower using daylight or tungsten filament bulbs thanthe light from an overhead projector because of the higher intensity ofthe overhead projector.

The Results are shown in FIG. 7

Example 12

A range of dyes have been tested as potential sensitising agent. Theyinclude: copper or iron complexes containing sulfonated phtalocyanineligands, silicon complex containing phtalocyanine ligand and rutheniumcomplexes containing bipyridyl or functionalised bipyridyl complexes(e.g: carboxylate, phosphonate) ligands and anions (e.g.: Cl, NCS) androse bengal.

A mixture of TiO₂ sol (1 ml, 1 g/L) and sensitizing agent (3 ml,c=3.4×10⁻⁵M) was stirred for about 1 minute using a rotamixer. Additionof gentian violet (0.05 ml, 0.03 wt/v %) was added as a target dye. AUV/Visible spectrum was taken at this stage. The vial containing themixture was placed onto an overhead projector (2 cm height from theglass, in order to reduce heat). UV/Visible spectra were used to observethe colour change over a period of time. All dyes tested decolourisedthe target dye gentian violet at different rates.

Example 13

Activity of Different TiO₂ sols.

Several TiO₂ sols have been tested for their activity includingcommercially available types from the Millennium Performance Chemicals,85 Avenue Victor Hugo, 92563 Rueil-Malmaison Cedex, France. A mixture ofTiO₂ sol (1 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) (4 ml,c=3.4×10⁻⁵M) was stirred for about 1 minute using a rotamixer. Additionof gentian violet (0.08 ml, 0.03 wt/v %) produced a purple colour. AUV/Visible spectrum was taken at this stage. The vial containing themixture was placed onto an overhead projector (2 cm height from theglass, in order to reduce heat). UV/Visible spectra were used to observethe colour change over a period of time. The results are summarised inTable 5.

TABLE 5 Activity of different sols. Time required to decolourise 0.08 mlof TiO2 sol source gentian violet (0.03 wt/v %) Kormann method >4 hoursJ. Woodhead Patent 1 h 30 mins Isopropoxide route 1 h 30 mins Millenniumsol in acidic medium 15 mins

Example 14

Activity of Different Sols in Basic Medium.

The TiO₂ sol (made from isopropoxide route) containing PVA (MW: 15,000)was prepared as follows. PVA (0.10 g, MW:15,000) was diluted in hotde-ionised water (50 ml) then allowed to cool to room temperature. Aknown amount of concentrated TiO₂ sol was added to the PVA solutionunder vigorous stirring. The volume was completed to 100 ml withde-ionised water. Final TiO2 concentration 1 g/L.

Basic System No PVA.

A mixture of TiO2 sol (isopropoxide route, 10 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) (40 ml,3.4×10⁻⁵M) was stirred using a stirrer hotplate. The pH was adjusted byaddition of a sodium hydroxide solution (0.1M) to pH 10. Gentian violet(0.08 ml, 0.03 wt/v %) was added to the mixture (volume used: 5 ml).

Basic System with PVA (1).

A mixture of TiO₂ sol containing PVA at pH 10.03 (1 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) also atpH 10.1 (4 ml, c=3.4×10⁻⁵M) was stirred for about 1 minute using arotamixer. The pH (9.85) was adjusted with a sodium hydroxide solution(0.1M) in order to get pH 10. Gentian violet (0.08 ml, 0.03 wt/v %) wasadded to the mixture.

Basic System with PVA (2).

A mixture of TiO2 sol containing PVA (10 ml) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) (40 ml,c=3.4×10⁻⁵M) was stirred using a stirrer hotplate. The pH was adjustedwith a sodium hydroxide solution (0.1M) to pH 10. Gentian violet (0.08ml, 0.03 wt/v %) was added to the mixture (volume used: 5 ml).

Millennium Basic System.

A mixture of Millennium TiO₂ sol in Basic medium(1 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)dichloride (4 ml,c=3.4×10⁻⁵M) was stirred for about 1 minute using a rotamixer. The pHwas adjusted with a sodium hydroxide solution (0.1 M) to pH 10.

Addition of gentian violet (0.05 ml, 0.03 wt/v %) produced a purplecolour in all systems. A UV/Visible spectrum was taken at this stage.The vial containing the mixtures were placed onto an overhead projector(2 cm height from the glass, in order to reduce heat). UV/Visiblespectra were used to observe the colour change over a period of time.

The Results are Summarised in table 6.

TABLE 6 Activity of TiO₂ sols in basic medium Time required todecolourise 0.08 ml TiO₂ sol source of gentian violet RemarksIsopropoxide route no PVA >4 hours Only stable for a short period oftime. Isopropoxide route with >4 hours PVA helped stabilised PVA (1) thesystem. Isopropoxide route with >4 hours PVA helped stabilised PVA (2)the system. Millennium in basic 1 h 10 min Stable sol. No need to mediumadd any surfactant.

Example 15

A solution containing a TiO2 sol (Millennium TiO2 sol in basic medium,10 g/L, 5.0 ml),tris(2,2′-bipyridyl4,4′-dicarboxylate)ruthenium(II)(dichloride) (1.8 ml,3.4×10⁻⁵M), Igepal® CO-720 (0.18 g) and de-ionised water (3.2 ml) wasstirred for few minutes using a rotamixer. The pH was adjusted to 10 byaddition of a sodium hydroxide solution (0.1M). The mixture was wrappedinto some aluminium foil and left standing overnight to equilibrate. Asolution containing a TiO2 sol (Millennium TiO2 sol in basic medium, 10g/L, 5.0 ml), Igepal® CO-720 (0.18 g) and de-ionised water (5.0 ml) wasstirred for few minutes using a rotamixer. The pH was adjusted to 10 byaddition of a sodium hydroxide solution (0.1M).

Thin films of these solutions were prepared by spin coating 0.1 ml ofthese solutions at 100 to 500 rpm on a clean glass microscope slide for80 seconds. The film was dried using a hot air gun and the processrepeated to give a total of 2 coats on the microscope slide. A secondslide was then prepared in exactly the same way. All slides were thenimmersed into a solution of 0.3% Gentian Violet in 20% ethanol for 5minutes. The slides were removed, rinsed with water to remove any excessstain and allowed to air dry. One slide was kept in total darkness andthe second was placed onto an overhead projector (Model Ensign. Lamp:24V-250 W-3860 lux). The purple colour on the films faded after 3 hours30 min. There was no change in the colour of the slide stored indarkness.

Example 16

The titania sols have been characterised by TEM (transmission electronmicroscopy). The samples were prepared by pipetting a few drops of thesol onto holey carbon films. Gold grids were used to avoid supportcorrosion. The microscope used was a Philips CM20, operated at 200 kV.The results are summarised in Table 7.

TABLE 7 TEM results of different source of titania. TiO₂ sol source TEMresults Prepared as per example 1 of GB 1412 Irregularly shaped titaniacrystallites mainly 937 (Woodhead) (peptised at 60° C.) anatase with asize of 10 nm. (A) Prepared as per example 1 of GB 1412 Irregularlyshaped titania crystallites mainly 937 (Woodhead) (peptised at roomanatase with a size of 10 nm. temperature) (B) Sensitised TiO₂ sol* (C)Irregularly shaped titania crystallites mainly anatase with a size of 10nm. Isopropoxide route (D) Sample had to be diluted. The titaniacrystallites were more tightly packed together. The crystallite size isvery small. The particles formed self- supporting films over holes onthe carbon film. Millennium sol Basic medium (E) Titania particlesappeared to be much more agglomerated than samples A and C. As a resultof this agglomeration, the particles formed self- supporting films overholes on the carbon film. *TiO2 sol (Prepared as per example 1 of GB1412 937 (Woodhead) 10 g/L, 5.0 ml),tris(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II)(dichloride) (1.8ml, 3.4 × 10⁻⁵M) and de-ionised water (3.2 ml)

Example 17

Activity of Stabilised Sensitised Titania Versus Titania Only.

TiO₂ Only with PVA.

The TiO₂ sol (made from isopropoxide route) containing PVA (MW: 15,000)was prepared as follows. PVA (0.10 g, MW:15,000) was diluted in hotde-ionised water (50 ml) then allowed to cool to room temperature. Aknown amount of concentrated TiO₂ sol was added to the PVA solutionunder vigorous stirring. The volume was completed to 100 ml withde-ionised water. Final TiO2 concentration 1 g/L.

A mixture of TiO₂ sol containing PVA (1 ml, 1 g/L) and de-ionised water(4 ml) was stirred for about 1 minute using a rotamixer. Gentian violet(0.08 ml, 0.03 wt/v %) was added to the mixture.

Sensitised TiO₂ with PVA-Acidic Medium.

The TiO₂ sol (made from isopropoxide route) containing PVA (MW15,000)was prepared as follows. PVA (0.10 g, MW 15,000) was diluted in hotde-ionised water (50 ml) then allowed to cool to room temperature. Aknown amount of concentrated TiO₂ sol was added to the PVA solutionunder vigorous stirring. The volume was completed to 100 ml withde-ionised water. Final TiO2 concentration 1 g/L. A mixture of TiO₂ solcontaining PVA (1 ml, 1 g/L) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) (4 ml,c=4.3×10⁻⁵M) was stirred for about 1 minute using a rotamixer. Gentianviolet (0.08 ml, 0.03 wt/v %) was added to the mixture.

Sensitised TiO₂ with PVA-Basic Medium.

A mixture of TiO2 sol containing PVA (10 ml) andtris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) (40 ml,c=3.4×10⁻⁵M) was stirred using a stirrer hotplate. The pH was adjustedwith a sodium hydroxide solution (0.1M) to pH 10. Gentian violet (0.08ml, 0.03 wt/v %) was added to the mixture (volume used: 5 ml).

A UV/Visible spectrum is taken at this stage. The vials containing themixtures were placed onto an overhead projector (2 cm height from theglass, in order to reduce heat). UV/Visible spectra were used to observethe colour change over a period of time. TiO2 without dye hassignificantly slower photocatalytic activity when compared withsensitised TiO₂

Example 18 Breakdown of 4-chlorophenol (halogenated pollutant) usingTiO2.

A microscope slide containing a thin film of sensitised TiO₂ was addedinto a solution of 4-chlorophenol (99+%, Aldrich, 8 ml, 10⁻⁴M). The vialcontaining the solution and the slide was placed onto an overheadprojector. The degradation of 4-chlorophenol was monitored usingUV/Visible analysis. A spectrum was taken over a period of time at maxof the 4-chlorophenol (=280 nm).

The absorbance at 280 nm was decreasing over time.

Example 19

Demonstration of the Photocatalytic Nature of TiO2 in Addition toSensitiser

A mixture of TiO₂ sol (5 ml, 10 g/L),tris(2,2′-dipyridyl-4,4′-dicarboxylate)ruthenium(II)(dichloride) (1.8ml, 3.4×10⁻⁵M) and de-ionised water (3.2 ml) was stirred for about 1minute using a rotamixer. Half of the volume of the mixture was used fortesting. A target dye, gentian violet (0.08 ml, 0.03 wt/v %) was addedto the mixture producing a blue-purple colour. A UV/Visible spectrum wastaken. The vial containing the mixture was placed onto an overheadprojector (2 cm height from the glass in order to reduce heat). AUV/Visible spectrum was taken over a period of time in order to observethe colour change. Once the spectrum showed no trace of target dye, thesame amount of gentian violet was added to the same mixture. The allprocess was repeated twice. The target dye gentian violet was stilldecolourising on the third addition but at a slower rate than on thefirst addition.

Example 20

Zinc oxide was prepared according to the method outlined by Bahnemann etal, J. Phys. Chem., (1987), 91, 3789. The oxide suspension (made bystirring the ZnO solid into a sodium hydroxide solution at pH9) was thensensitised with 4,4′-dicarboxa late,tris(2,2′bibyridyl)Ru(II)dichlorideaccording to the method outlined in previous examples. Gentian violetdye was added to both the sensitised sample and the non-sensitisedcontrol sample, and the UV/visible spectrum was recorded as a functionof time under illumination with white light (5,000 lux). The resultsdemonstrate that the absorption peak associated with Gentian Violetdecreases faster with the sensitised ZnO compared to the control.

Example 21

The methods described below offer general synthetic pathways tosensitising agents specifically designed to function in combination withsemiconductors where the desired operating condition is such that theun-coated semiconductor surface presents a significant number ofadsorption sites with a negative charge. Typical positively chargedgroups for use as binding sites include, but are not limited to R₄N+groups and R₄P+ groups, where R is as hereinbefore described

Novel Dyes Based on Terpyridyl

Terpyridyl-based sensitisers with phosphonate chelating ligands(Graetzel at al, WO 95/29924) and with other ligand types (Graetzel etal, WO 94/0449) have been used in conjunction with titania indye-sensitised solar cells. The terpyridyl group of general formula Ican be synthesised with e.g. R1 as a positively charged unit. Oneexample where R5-7 of formula II are methyl, is synthesised according toprocedures where the intermediate is made by the method outlined inRecl. Trav. Chim. Pays. Bas, 1959, v78, 408. This nitrated aryl group isthen changed into the terpyridyl unit by the method outlined by McWhinneet al (J Organoetallic chem., 1968, v11, 499). The nitro group is thenreduced to the amine by hydrazine hydrate under Pd/C catalysis followedby reaction with excess methyl iodide to form the quaternary nitrogenterpyridyl ligand desired.

Yet another variant of the positively charged terpyridyl molecule[described by general formula I] can be synthesised by reacting2-acetylpyridine with 4-nitrobenzaldehyde in base followed by ringclosure with ammonium acetate according to methods outlined by EConstable et al (J Chem Soc Dalton Trans, 1992, 2947), followed byreduction of the nitro group to the amine and quaternisation asdescribed previously to form a compound described by formula I with R2and R 3 as hydrogen and R1 as

Yet another general preparative method for a terpyridal group of generalformula I is based on derivatising the structure below (Potts et al,JACS 1987, v109, 3961) by oxidising it to the carboxylic acid by e.g.the methodology outlined by Dodd et al (Synthesis (1993), V3, 295).Amination of the carboxylate is then carried out by standard proceduresoutlined in e.g. Chem Rev. (1981) V81, p589.

Bipyridyls

The synthesis of a compound of general formula III with R8,9=NH2 isdescribed in (JACS 1958, V80, 2745) and (J Chem Soc Perkin Trans 2,1996, 613) and a compound of formula III can be quarternised by thepreceding methods outlined for terpyridyls.

Phthalocyanines

Phthalocyanine dyes can be synthesised with amine nitrogen groups bye.g. Buchwald ammination of halide precursors to produce outer-ringderivatives such as (J Org Chem 2000, V65, 1158), the amine groups ofwhich are then quarternised.

Tetra-aza-annulenes (TADAs)

TADAs of the general formula V can be derivatised at R1, R2, R3, and R4by the general methods outlined above.

Dimers

The bipyridyl compoundstris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II)dichloride andtris(2,2′-bipridyl)ruthenium (II) dichloride can be dimerised usingpyrazine derivatives such as pyrazine, pyrimidine and 4,4′-bipyridyllinking ligands according to procedures detailed in (E A Seddon & K RSeddon, The Chemistry of Ruthenium, Elsevier, New York 1984, p 436).

1. A method of cleaning a surface comprising cleaning the surface by:(1) coating said surface with a composition comprising a photocatalystselected from the group consisting of titania and zinc oxide and aruthenium bi- or ter-pyridyl or phenanthroline-based sensitizer, and (2)exposing the surface to visible or infra-red light.
 2. A methodaccording to claim 1, wherein the sensitizer used comprises a terpyridylgroup of general formula I:

wherein at least one of R1, R2 and R3 are positively charged groupswhich has the general formula II shown below:

wherein R5-R7 are any one or more of the following groups: hydrogen,halogen, amine, alkyl, aryl, arylalkyl, alkoxy, heterocyclic groups, orderivatives thereof, including acid and ester derivatives, any of whichmay be branched or unbranched, substituted or unsubstituted.
 3. A methodaccording to claim 2, wherein the sensitizer used includes a bipyridylgroup of the general formula III:

wherein R8 and R9 can be the same or different and is any one or more ofthe following groups: hydrogen, halogen, amine, alkyl, aryl, arylalkyl,alkoxy, heterocyclic groups, or derivatives thereof, including acid andester derivatives, any of which may be branched or unbranched,substituted or unsubstituted, R2 may be the same or different from R3and is any one or more of the following groups: hydrogen, halogen,amine, alkyl, aryl, arylalkyl, alkoxy, heterocyclic groups, orderivatives thereof, including acid and ester derivatives, any of whichmay be branched or unbranched, substituted or unsubstituted.
 4. A methodaccording to claim 3 wherein the sensitiser is selected from the groupconsisting of tris (2,2′-dipyridyl-4,4′dicarboxylate) ruthenium (II) andtris (2,2′-dipyridyl) dichlororuthenium (II) and their dimers.
 5. Amethod according to claim 4, wherein the sensitiser is tris(2,2′-dipyridyl-4,4′-dicarboxylate) ruthenium (II) (dichloride).
 6. Amethod according to claim 1, wherein the photocatalyst is selected fromthe group consisting of a nanocrystalline titanium dioxide sol and zincoxide sol.
 7. A method according to claim 1, wherein the composition isin the form of an acidic liquid suspension or sol having a pH less than7.
 8. A method according to claim 7, wherein the composition has a pHless than
 5. 9. A method according to claim 1, wherein the compositionis in the form of a liquid suspension or sol, having a pH of 7 or more.10. A method according to claim 9, wherein the composition is in theform of a liquid suspension or sol, having a pH of 9 or more.
 11. Amethod according to claim 1, wherein the composition further comprises adoping agent.
 12. A method according to claim 1, wherein thephotocatalyst comprises no more than 1% w/v of the composition.
 13. Amethod according to claim 1, wherein the photocatalyst comprises no morethan 0.1% w/v of the composition.
 14. A method according to claim 1,wherein the sensitiser is present in an amount no more than 0.1% w/v ofthe composition.
 15. A method of cleaning a surface comprising cleaningthe surface by depositing a dry functional residue on a surface, saidresidue deposited from a composition, said composition comprising aphotocatalyst selected from titania and zinc oxide, wherein thephotocatalyst is sensitised by a ruthenium bi- or ter-pyridyl orphenanthroline-based sensitiser and said sensitiser causes activation byvisible or infra-red light.